The present invention relates to a conformal coating material (hereinafter referred to as coating material) that forms an insulating moisture-proof membrane, in which cracking and peeling from the substrate tend not to occur and which does not apply excessive stress to the lead pin of electronic parts.
Due to widespread use of computers, circuit boards loaded with electronic parts have come to be used in household electric products and automobiles. Also, solvent coating materials having moisture resistance and insulating properties have come to be applied, in order to protect circuit boards from sudden temperature change, humidity, water and dust.
In recent years, along with increase in social awareness regarding environmental problems, a coating material has been developed, in which ethyl acetate, methyl acetate and butyl acetate are used instead of aromatic hydrocarbon solvents such as toluene and xylene, which are high in toxicity (see JP-A-2002-146266). However, this coating material lets out a great deal of solvent when applied and therefore, has many problems such as danger of fire, foul odor generated by the evaporated solvent and health management of workers to poisoning. Furthermore, depending on the locational conditions of the working area, measures against the Foul Smell Control Law may become necessary.
Also, water-based coating materials that do not contain a solvent have been considered. However, because film-forming properties and durability of the coating film are not sufficient, at present, coating materials contain 5 to 10% of a solvent. In such water-based coating materials containing a solvent, when the Young's modulus is low, the membrane remains sticky and problems occur, such as adhesion of dirt and blocking when handling. Also, coating materials having less stickiness are not sufficient, as these have large Young's modulus and apply large load to the substrate. As a result, peeling of solder and deformation of the lead pin may occur, due to expansion and contraction of the membrane caused by increase and decrease in the environmental temperature.